Secondary batteries are used in electric vehicles and hybrid electric vehicles, for example, to supply their electric motors for driving the vehicles with electric power (electric discharge) and to store the electric energy obtained from the electric motors that functions as a generator that changes brake energy into the electric power or from a power source equipment on the ground.
In this case, in order to maintain the states of the secondary batteries to be optimal over the long term, it is needed to always monitor the states of the batteries such as the states of charge (SoC) and thereby carry out its battery management.
The states of the batteries, however, cannot be directly estimated when the batteries employ secondary batteries because the discharge and the charge occur under chemical action.
Accordingly, in order to estimate the states of the batteries, a battery equivalent circuit model is constructed, which is then modified by feed-backing the error between the outputs obtained when the input signal is inputted to the model and the battery. Then parameters of the model are estimated, and the state of charge and others are estimated based on information obtained from the model.
In the parameter estimation, in order to remove the parameter-estimation errors due to parameter-estimation delay generated in a case where circuit voltage of the battery is small when the state of charge varies, an influence of noise, or the deterioration of trailing of an adaptive digital filter when true values of the parameters varies at rapid speed, it is known that the current and the voltage respectively obtained by a current sensor and a voltage sensor are preprocessed.
One of the conventional parameter estimation devices is disclosed in the patent document 1 for example, where an adjustment gain of the active digital filter is set to be small, or a time constant of a low-pass filter is set to be large in an area where the circuit-voltage variation is large when the state of charge varies, while the adjustment gain of the adaptive digital filter is set to be large or the time constant of the low-pass filter is set to be small in an area where the circuit-voltage variation is small when the state of charge varies.
The other conventional parameter estimation device is disclosed in the patent document 2 for example, where a first state of charge is obtained based on estimation results of an adaptive digital filter and a second state of charge is obtained by using a current integration method (a Coulomb counting method). Then the first state of charge is selected when the positive and negative symbols of the current turn over, and then the second state of charge is selected after only charge current or only discharge current runs for a predetermined time starting from the above turn-over time. At this time, the detected current and voltage are preprocessed through a low-pass filter process and an approximate-differential filter process.